790 PHYSIOLOGY 



from other parts of the body ; the second, and probably the more important 

 one, is a change in the relation of the fat to the protoplasm of the cell. 



It was long stated that the fat of milk was not increased by feeding with 

 fats, but only by feeding with proteins. More recent researches have given 

 contrary results. The dependence of the composition of milk fat on the 

 composition of the fat present in the body or administered in the food is 

 shown by the fact that cows fed on oilcake may produce a butter which 

 is useless for commercial purposes owing to its low melting-point. In one 

 experiment, when a cow was fed on linseed oil, the iodine number of the 

 milk fat rose from 30, its normal figure, to 704. After the introduction 

 of iodine fat subcutaneously, iodine fats are found in the milk. In another 

 experiment a bitch which had been fed with mutton suet and had deposited 

 in its tissues a fat of high melting-point produced a milk the iodine number 

 of which was the same as that of the mutton suet. In this case the fat 

 of the milk had evidently been derived from the tissues, since during the 

 lactation the animal was being fed on meat which was poor in fat. The 

 same dependence of fatty secretion on diet has been found in geese, where 

 the composition of the oil secretion of the feather glands has been altered 

 by giving abnormal fats, such as sesame oil, with the food. 



We must conclude that the protein of the food does not give rise to 

 fat in the body. A nearer consideration of the composition of the proteins, 

 taken in connection with our discussion as to the mechanism by means of 

 which the fat is built up in the body, might help to account for this fact. 

 The fatty acids formed by the disintegration of proteins are chiefly the 

 lower acids of the series, such as acetic and propionic, which would undergo 

 rapid oxidation in the body. Butyric acid has not yet been found among 

 the products of disintegration of tjie proteins, and the 6-carbon acid, 

 derived from leucine, is not the normal acid, but is a branched chain, viz, 

 isobutyl-acetic acid. 



THE UTILISATION OF FATS IN THE BODY 



The constant presence of fat, and bodies allied to fat, in protoplasm, 

 from whatever source obtained, suggests that these substances can enter 

 directly into the chemical changes on which the life of the cell depends 

 and that they play an essential part in vital phenomena. The direct 

 utilisation of fat for the needs of the body is also indicated by the results 

 of experiments on man and the lower animals. After a few days' starva- 

 tion the body may be regarded as practically free from stored carbohydrate. 

 The sole source of the energy which is evolved under these circumstances 

 must be fats and proteins, and it is possible to determine by an estimation 

 of the nitrogen output the exact fraction of the total energy evolved which 

 is to be ascribed to protein metabolism. Thus in the case of Cetti, the 

 professional faster, it was found that the nitrogenous metabolism per unit 

 of body weight remained fairly constant between the fifth and tenth days 

 of starvation, and corresponded to an average of 1 grm. of protein per 

 kilo body weight daily. In order to convert this amount of protein into 



